It is known to equip a vehicle 100 (FIG. 1) with a catalytic converter designed to process the exhaust gases 106 emitted by its engine 102, and this catalytic converter may be of the nitrogen oxide (NOx) trap 104 type.
In this case, this trap 104 consists of materials with an affinity for nitrogen oxides, in order, at first, to retain the latter when the gases 106 pass through the trap 104 and subsequently to make it possible to reduce their nitrogen (N2) content. In fact, such a trap 104 alternates between two modes of operation, which are characteristic of the nitrogen oxide trap and are described in detail below:
A first mode of operation corresponds to a storage of the nitrogen oxides, during which the trap 104 collects the latter in the exhaust gases 106.
This mode corresponds to a so-called “lean” operation of the engine such that the oxygen is present in excess compared to the fuel. In this case, the richness r of the mixture, which is equal to the ratio of the quantity of fuel to the quantity of oxygen, is less than one.
During this first mode of operation, the storage of nitrogen oxides is limited by the storage capacity of the trap 104, which can be defined as the maximum mass Max of nitrogen oxides that this trap 104 can collect.
By considering the entering NOx mass Ment(t) and the exiting NOx mass Mexit(t) at a given instant t, the storage efficiency E(t) of the trap 104 can be defined as the difference between the entering mass Ment(t) and the exiting mass Mexit(t) of nitrogen oxides actually stored in the trap 104, divided by the nitrogen oxide entering mass Ment(t).
Such a definition thus corresponds to the following formula (1):
                              E          ⁡                      (            t            )                          =                                                            M                ent                            ⁡                              (                t                )                                      -                                          M                exit                            ⁡                              (                t                )                                                                        M              ent                        ⁡                          (              t              )                                                          (        1        )            
This formula (1) reflects the decrease in the efficiency E(t) of a nitrogen oxide trap as the nitrogen oxide mass M(t) stored tends toward the maximum nitrogen oxide mass Max(T) that can be stored.
This decrease is empirically measurable as is shown in FIGS. 2a and 2b, which plot the efficiency E(t) (ordinate 200, in percentage) of the nitrogen oxide trap 104 against the nitrogen oxide mass (abscissa 202, in grams) stored in this trap 104.
Furthermore, FIG. 2a also shows that the efficiency E(t) of the nitrogen oxide trap also decreases when the quantity of sulfur (S) collected by the trap increases in the latter, this decrease being due to a lowering of the storage capacity of the trap.
In fact, the efficiencies E0, E1, E2, E3 and E4, measured for traps having a sulfur content close to 0, 1, 2, 3 and 4 grams per liter, respectively, are decreasing for a same stored quantity of nitrogen oxides.
This is why it is necessary to carry out operations for the removal of sulfur from storage at regular intervals in order to recover the storage capacity.
However, such operations of removing sulfur have the drawback of irreversibly reducing the storage capacity, and therefore the efficiency, of the trap on the long term as shown below by means of FIG. 2b, which shows efficiencies E′0, E′1, E′2, E′3 and E′4 measured for traps having undergone increasing numbers of sulfur storage/removal cycles (0, 5, 10, 18 and 30, respectively), and the higher this number of cycles, the lower these efficiencies are.
In fact, these sulfur removal operations subject the trap to high temperatures (greater than 600° C.) for a period generally ranging from 4 to 20 minutes, which brings about the degradations, called thermal aging, of the catalytic phase of the trap.
This is why it is known to check the frequency of the removal of sulfur of a trap by determining the quantity of sulfur received by the latter from the consumption of the vehicle and from a sulfur content attributed to the fuel.
A second mode of operation of the trap 104 corresponds to the nitrogen (N2) reduction of the nitrogen oxides collected by this trap, the latter reacting with the reducers (HC: hydrocarbons, CO: carbon monoxide and H2: hydrogen) supplied by the engine 102 via the exhaust gases 106.
For this, the quantity of reducers supplied to the trap 104 is increased by means of a so-called “rich” operation of the engine 102, the quantity of fuel introduced in the engine being greater than the quantity of oxygen in relation to stoichiometric conditions, and the richness r of the mixture is greater than 1.
This mode of removal requires a good determination of the quantity of nitrogen oxides present in the trap 104 in order to actuate the engine in such a way that it supplies the optimal richness, called r, defined as the ratio between the quantity of oxygen (oxidant) and the quantity of reducers (HC, CO and H2) in the exhaust gases.
Actually, if there is a lack of oxygen compared to the reducers, the latter are emitted into the environment, while the reduction of nitrogen oxides would be incomplete due to a lack of reducers if an excess of oxygen was present.
This determination is currently made by means of an operation model of the trap 104, which aims at predetermining the storage capacity of the latter as a function, for example, of the number of times nitrogen oxides or sulfur have been removed, in order to optimally decide on new removal operations.